A semiconductor device with protective functions having a circuit construction, for example shown in FIG. 11, is conventionally known. The semiconductor device with protective functions shown in FIG. 11, which is one of three-terminal semiconductor devices, is provided with an external drain terminal (first main terminal) D, an external source terminal (second main terminal) S and an external gate terminal (control terminal) G. The semiconductor device is inserted between a power source and a load. In the semiconductor device, a current flowing between the external drain terminal D and the external source terminal S is controlled in accordance with a control voltage applied between the external gate terminal G and the external source terminal S.
The semiconductor device shown in FIG. 11 is provided with a main MOSFET 1 which is turned to ON or OFF in accordance with the control voltage and which is inserted between the external drain terminal D and the external source terminal S, and a protective circuit 3′ for protecting the main MOSFET 1. Hereupon, the protective circuit 3′ is provided with a protective MOSFET 5′ inserted between the gate electrode and the source electrode of the main MOSFET 1, a diode array 31 acting as a temperature detector which is thermally connected with the main MOSFET 1, another MOSFET 35 connected to the diode array 31 at each of the gate electrode and the source electrode thereof, further MOSFETs 37 and 38 connected to each other in such a manner that the drain electrode of the MOSFET 37 is connected to the gate electrode of the MOSFET 38, and a further MOSFET 36 inserted between the drain electrode and source electrode of the MOSFET 37, the gate electrode of the MOSFET 36 being connected to the drain electrode of the MOSFET 35. According to the circuit construction, when the temperature detected by the diode array 31 rises to or over a predetermined value, the main MOSFET 1 is shut, and then the shut state of the main MOSFET 1 is maintained till the input to the external gate terminal G is reset. That is, when the temperature detected by the diode array 31 rises to or over the predetermined value, the gate voltage of the MOSFET 35 becomes lower than a threshold voltage. In consequence, the MOSFET 35 becomes OFF, the MOSFET 36 becomes ON, and each of the MOSFETs 37 and 38 becomes OFF. As the result, the protective MOSFET 5′ becomes ON so that the gate electrode and source electrode of the main MOSFET 1 short-circuit, and then the main MOSFET 1 becomes OFF.
In the conventional semiconductor device with protective functions described above, the protective circuit 3′ operates using the voltage applied to the gate electrode of the MOSFET 1 as its power source. Accordingly, it is necessary to supply the power to both of the protective circuit 3′ and the main MOSFET 1. In that case, for example, there is required a driver which can supply a gate driving current of at least 0.1 mA. In consequence, there occurs such a disadvantage that it is difficult to drive the semiconductor device using electromotive force of a photovoltaic element which can supply only a very small gate driving current, or to drive the semiconductor device using power obtained by rectifying an alternating signal which is transmitted through capacitors.
Meanwhile, in the conventional semiconductor device with protective functions described above, if the temperature detected by the diode array 31 rises to or over the predetermined value when the main MOSFET 1 is overheated, the main MOSFET 1 is shut. The shut state of the main MOSFET 1 is maintained till the input to the external gate terminal G is reset. Therefore, when the semiconductor device is restarted (returned), the control voltage applied to the external gate terminal G is required to be reset as 0 V.
Hereupon, it is also possible to perform two-directional switching by providing two semiconductor devices with protective functions shown in FIG. 11, and then commonly connecting the external source terminals S to each other while commonly connecting the external gate terminals G to each other. However, in this case, it is feared that when the current flows from the external source terminal S to the external drain terminal D, the protective MOSFET 5′ of the protective circuit 3′ may operate in error.